Nanosecond laser ablation and deposition of silicon

Nanosecond-pulsed KrF (248 nm, 25 ns) and Nd:YAG (1064 nm, 532 nm, 355 nm, 5 ns) lasers were used to ablate a polycrystalline Si target in a background pressure of <10⁻⁴ Pa. Si films were deposited on Si and GaAs substrates at room temperature. The surface morphology of the films was characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Round droplets from 20 nm to 5 μm were detected on the deposited films. Raman Spectroscopy indicated that the micron-sized droplets were crystalline and the films were amorphous. The dependence of the properties of the films on laser wavelengths and fluence is discussed.     

Effects of laser fluence on the structural properties of pulsed laser deposited ruthenium thin films

Ruthenium (Ru) has received great interest in recent years for applications in microelectronics. Pulsed laser deposition (PLD) enables the growth of Ru thin films at low temperatures. In this paper, we report for the first time the characterization of pulsed laser deposited Ru thin films. The deposition processes were carried out at room temperature in vacuum environment for different durations with a pulsed Nd:YAG laser of 355-nm laser wavelength, employing various laser fluences ranging from 2 J/cm² to 8 J/cm². The effect of the laser fluence on the structural properties of the deposited Ru films was investigated using surface profilometry, scanning electron microscopy (SEM), and X-ray diffraction (XRD). Ru droplets, some spherical in shape and some flattened into round discs were found on the deposited Ru. The droplets were correlated to ripple formations on the target during the laser-induced ejection from the target. In addition, crystalline Ru with orientations of (100), (101), and (002) was observed in the XRD spectra and their intensities were found to increase with increasing laser fluence and film thickness. Grain sizes ranging from 20 nm to 35 nm were deduced using the Scherrer formula. Optical emission spectroscopy (OES) and energy-dispersive X-ray spectroscopy (EDS) show that the composition of the plume and the deposited Ru film was of high purity.     

Synthesis of ZnO thin films by 40 ps @ 532 nm laser pulses

The synthesis by pulsed laser deposition of ZnO thin films with a Nd:YAG laser system delivering pulses of 40 ps @ 532 nm is reported. The laser beam irradiated the target placed inside a vacuum chamber evacuated down to 1.33×10⁻¹ Pa. The incident laser fluence was of 28 J/cm² in a spot of 0.1 mm². The ablated material was collected onto double face polished (111) Si or quartz wafers placed parallel at a separation distance of 7 mm. The AFM, SEM, UV-Vis, FT-IR and absorption ellipsometry results indicated that we obtained pure ZnO films with a rather uniform surface, having an average roughness of 37 nm. We observed by SEM that particulates are present on ZnO film surface or embedded into bulk. Their density and dimension were intermediary between particulates observed on similar structures deposited with fs or ns laser pulses. We noticed that the density of the particulates is increasing while their average size is decreasing when passing from ns to ps and fs laser pulses. The average transmission in the UV-Vis spectral region was found to be higher than 85%.     

Double-peak droplet mass distribution observed during sub-ps laser ablation of Si targets

The angular distribution of the ablated material was studied during sub-ps Si laser ablation deposition using a special hemicylindrical substrate holder and different laser fluences ranging between 0.4 and 1.7 J/cm². Scanning electron microscopy analysis of the deposited films showed that, independent of the fluence, the distribution of the deposited droplets presents two maxima. The first maximum corresponds to the average plume deflection angle value due to the local surface orientation produced by the preferential etching process. The second maximum is observed approximately at 45° with respect to the normal of the target surface, and is related to the phase explosion products that expand along the incident laser beam direction. The investigation of the twofold distribution of the sub-μm size deposited droplets is important to improve the quality of the deposited coatings. PACS 81.15.Fg; 68.55.Jk; 79.20.Ds     

Formation of conical microstructures upon laser evaporation of solids

The formation and development of the large-scale periodic structures on a single crystal Si surface are studied upon its evaporation by pulsed radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The development of structures occurs at a high number of laser shots (∼10⁴) at laser fluence of 1–2 J/cm² below optical breakdown in a wide pressure range of surrounding atmosphere from 1 to 10⁵ Pa. The structures are cones with angles of 25, which grow towards the laser beam and protrude above the initial surface for 20–30 μm. It is suggested that the spatial period of the structures (10–20 μm) is determined by the capillary waves period on the molten surface. The X-ray diffractometry reveals that the modified area of the Si substrate has a polycrystalline structure and consists of Si nanoparticles with a size of 40–70 nm, depending on the pressure of surrounding gas. Similar structures are also observed on Ge and Ti. Received: 12 February 2000 / Accepted: 28 March 2000 / Published online: 20 June 2001     

Laser-assisted generation of self-assembled microstructures on stainless steel

Utilising a Nd:YVO₄ laser (wavelength of 532 nm, pulse duration of 8 ns, repetition rate of 30 kHz) and a Nd:YAG laser (wavelength of 1064 nm, pulse duration of 7 ns, repetition rate of 25 kHz), it was found that during the pulsed laser ablation of metal targets, such as stainless steel, periodic nodular microstructures (microcones) with average periods ranging from ∼30 to ∼50 μm were formed. This period depends on the number of accumulated laser pulses and is independent of the laser wavelength. It was found that the formation of microcones could occur after as little as 1500 pulses/spot (a lower number than previously reported) are fired onto a target surface location at laser fluence of ∼12 J/cm², intensity of ∼1.5 GW/cm². The initial feedback mechanism required for the formation of structures is attributed to the hydrodynamic instabilities of the melt. In addition to this, it has been shown that the structures grow along the optical axis of the incoming laser radiation. We demonstrate that highly regular structures can be produced at various angles, something not satisfactorily presented on metallic surfaces previously. The affecting factors such as incident angle of the laser beam and the structures that can be formed when varying the manner in which the laser beam is scanned over the target surface have also been investigated.     

Processing of C<Subscript>60</Subscript> thin films by Matrix-Assisted Pulsed Laser Evaporation (MAPLE)

Thin films of fullerenes (C₆₀) were deposited onto silicon using matrix-assisted pulsed laser evaporation (MAPLE). The deposition was carried out from a frozen homogeneous dilute solution of C₆₀ in anisole (0.67 wt%), and over a broad range of laser fluences, from 0.15 J/cm² up to 3.9 J/cm². MAPLE has been applied for deposition of fullerenes for the first time and we have studied the growth of thin films of solid C₆₀. The fragmentation of C₆₀ fullerene molecules induced by ns ablation in vacuum of a frozen anisole target with C₆₀ was investigated by matrix-assisted laser desorption/ionization (MALDI). Our findings show that intact fullerene films can be produced with laser fluences ranging from 0.15 J/cm² up to 1.5 J/cm².     

Effect of laser fluence on the deposition and hardnessof boron carbide thin films

We deposited amorphous thin films of boron carbide by pulsed laser deposition using a B₄C target at room temperature. As the laser fluence increased from 1 to 3 J/cm², the number of 0.25–5 μm particulates embedded in the films decreased, and the B/C atomic ratio of the films increased from 1.8 to 3.2. The arrival of melt droplets, atoms, and small molecular species depending on laser fluence appeared to be involved in the film formation. In addition, with increasing fluence the nanoindentation hardness of the films increased from 14 to 32 GPa. We believe that the dominant factor in the observed increase in the films’ hardness is the arrival of highly energetic ions and atoms that results in the formation of denser films. Received: 23 March 2001 / Accepted: 1 July 2001 / Published online: 2 October 2001     

Preparation and characterization of pulsed laser deposition (PLD) SiC films

Si K-edge XAFS was used to characterize a stoichiometric SiC film prepared by pulsed KrF laser deposition. The film was deposited on a p-type Si(1 0 0) wafer at a substrate temperature of 250 °C in high vacuum with a laser fluence of ∼5 J/cm². The results reveal that the film contains mainly a SiC phase with an amorphous structure in which the Si atoms are bonded to C atoms in its first shell similar to that of crystalline SiC powder but with significant disorder.     

Waveguides and quasi-waveguides based on pyrromethene 597-doped poly(methyl methacrylate)

The characteristics of amplified spontaneous emission (ASE) from asymmetric planar waveguides and quasi-waveguides consisting of thin films of poly(methyl methacrylate) incorporating lasing dye pyrromethene 597 deposited onto quartz and glass substrates, respectively, are investigated. The variable stripe length and moving constant stripe methods, together with appropriate theoretical expressions which take into account gain saturation and a simple model based on a four-level laser, allow for obtaining the net gain coefficients as a function of pump intensity, losses, pump thresholds for the onset of ASE, effective stimulated emission cross sections, pump saturation intensities, and saturation lengths. Net gain coefficients of up to 84±3 cm⁻¹ at a pump intensity of 404 kW/cm² (28 μJ/pulse) for quasi-waveguides and up to 59±6 cm⁻¹ at a pump intensity of 360 kW/cm² (25 μJ/pulse) for waveguides were obtained, with pump thresholds of 15.7 kW/cm² (1.1 μJ/pulse) and 6.3 kW/cm² (0.43 μJ/pulse), respectively. When waveguides 8 μm thick were irradiated with pulses of 200 kW/cm² at 10 Hz repetition rate, the ASE remained at 79% of its initial value after 1000 pump pulses in the same position of the sample. In quasi-waveguides 10 μm thick, the emission remained at 82% of the initial value under the same conditions.     

Localized insulator-conductor transformation of graphene oxide thin films via focused laser beam irradiation

By scanning a focused laser beam over graphene oxide (GO) film deposited on SiO₂/Si substrates, conductive strips as small as 1 μm can be patterned directly either as a channel in the insulating matrix, or as a stand-alone micro belt. The conductivity was increased by at least two orders of magnitude with the mobility estimated in the range of 1–10 cm²/V s. Raman mapping and X-ray photoelectron spectroscopy studies demonstrated the reduction of GO in the laser-irradiated area. The conductance of the patterned channel was independent of the change in oxide-electrode contact resistance of the graphene, and increased linearly with increasing channel width. Increasing irradiation power by repeated scanning initially increased the conductivity of the irradiated area and saturated at a conductivity of ∼36 S/cm. Partial oxidative burning combined with photothermal reduction was identified as the underlying mechanism for the enhancement of the conductivity after laser irradiation on the GO film. Oxidative burning can be controlled by varying the film thickness and laser power.     

Aluminum-assisted crystallization and <Emphasis Type="Italic">p</Emphasis>-type doping of polycrystalline Si

Aluminum-doped p-type polycrystalline silicon thin films have been synthesized on glass substrates using an aluminum target in a reactive SiH₄+Ar+H₂ gas mixture at a low substrate temperature of 300 °C through inductively coupled plasma-assisted RF magnetron sputtering. In this process, it is possible to simultaneously co-deposit Si–Al in one layer for crystallization of amorphous silicon, in contrast to the conventional techniques where alternating metal and amorphous Si layers are deposited. The effect of aluminum target power on the structural and electrical properties of polycrystalline Si films is analyzed by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and Hall-effect analysis. It is shown that at an aluminum target power of 100 W, the polycrystalline Si film features a high crystalline fraction of 91%, a vertically aligned columnar structure, a sheet resistance of 20.2 kΩ/□ and a hole concentration of 6.3×10¹⁸ cm⁻³. The underlying mechanism for achieving the semiconductor-quality polycrystalline silicon thin films at a low substrate temperature of 300 °C is proposed.     

Fabrication and characteristics of a surface acoustic wave UV sensor based on ZnO thin films grown on a polycrystalline 3C-SiC buffer layer

Zinc oxide (ZnO) thin films were deposited onto a polycrystalline (poly) 3C-SiC buffer layer for surface acoustic wave (SAW) ultraviolet (UV) sensing using a magnetron sputtering system. X-ray diffraction (XRD) and photoluminescence (PL) spectra showed that the ZnO film grown on 3C-SiC/Si had a dominant c-axis orientation, a lower residual stress, and higher intensity of luminescence at 380 nm of ZnO thin film. The SAW resonator UV detector were fabricated on ZnO/Si structures with a 3C-SiC buffer layer. The SAW resonator exposed under UV illumination had a linear response with sensitivity of 85 Hz/(μW/cm²) in ZnO/3C-SiC/Si structures, as compared to 25 Hz/(μW/cm²) in ZnO/Si structures with UV intensity varied until 600 μW/cm².     

Millimeter-long nanofibers of PMMA spun at super-high speed by ablation with a single pulse of a KrF excimer laser

A new form of matter removal in laser ablation is reported. Polymethylmethacrylate (PMMA) nanofibers are obtained when a PMMA target is irradiated with a single pulse of a KrF excimer laser, whose beam is sharply imaged on a square of side the order of 140 μm, so that a strong intensity gradient is produced. The fluence threshold at which fibers appear, 3 J/cm², is much larger than the ablation threshold, approximately 0.8–1 J/cm². Above this fluence, the melt depth is then large enough and the temperature profile is such that explosive boiling is obtained. The model suggests an expulsion of energetic droplets from the intense pressure of the plume to the exterior of the spot. For the transient melt of a polymeric viscoelastic liquid resulting from UV-laser excitation, such droplets provide the heads of the jets pulled from the melt bath, giving rise, after solidification, to nanofibers. The speed of fiber spinning is extremely high (∼800 m/s) and unusual properties of the laser-produced nanofibers may be expected. Received: 16 April 2002 / Accepted: 17 April 2002 / Published online: 19 July 2002     

An investigation into the effects of high laser fluence on hydroxyapatite/calcium phosphate films deposited by pulsed laser deposition

Pulsed laser deposited mixed hydroxyapatite (HA)/calcium phosphate thin films were prepared at room temperature using KrF laser source with different laser fluence varying between 2.4 J/cm² and 29.2 J/cm². Samples deposited at 2.4 J/cm² were partially amorphous and had rough surfaces with a lot of droplets while higher laser fluences showed higher level of crytallinity and lower roughness of surfaces of obtained samples. Higher laser fluences also decreased ratio Ca/P of as-deposited samples. X-ray photoelectron spectroscopy (XPS) revealed traces of carbonate groups in obtained samples, which were removed after thermal annealing. The decomposition of HA into TCP was observed to start at about 400 °C. The formation of new crystalline phase of HA was found after annealing as well. The cracks observed on surface of sample deposited at 29.2 J/cm² after annealing indicated that the HA/ calcium phosphate films deposited at higher laser energy densities were probably more densed.     

Formation of highly organised,periodic microstructures on steel surfaces upon pulsed laser irradiation

We present results on the growth of highly organised, reproducible, periodic microstructure arrays on a stainless steel substrate using multi-pulsed Nd:YAG (wavelength of 1064 nm, pulse duration of 7 ns, repetition rate of 25 kHz, beam quality factor of M ²∼1.5) laser irradiation in standard atmospheric environment (room temperature and normal pressure) with laser spot diameter of the target being ∼50 μm. The target surface was irradiated at laser fluence of ∼2.2 J/cm² and intensity of ∼0.31×10⁹ W/cm², resulting in the controllable generation of arrays of microstructures with average periods ranging from ∼30 to ∼70 μm, depending on the hatching overlap between the consecutive scans. The received tips of the structures were either below or at the level of the original substrate surface, depending on the experimental conditions. The peculiarity of our work is on the utilised approach for scanning the laser beam over the surface. A possible mechanism for the formation of the structures is proposed.     

Self-assembled organic monolayers as high-resolution resists in rapid nonlinear processing with single femtosecond laser pulses

Femtosecond laser patterning of alkanethiol monolayers on gold-coated silicon substrates at λ=800 nm, τ<30 fs and ambient conditions has been investigated. Single-pulse processing allows one to selectively remove the organic coating. Subsequently, pattern transfer into the gold film via wet etching in ferri-/ferrocyanide solution is achieved. As demonstrated, burr-free patterning can be carried out over an extremely wide range of laser pulse fluences from above 2 J/cm² down to 0.5 J/cm². Moreover, at low fluences, sub-wavelength processing down to λ/5 is feasible. In particular, at a 1/e laser spot diameter of about 1 μm, holes with diameters of 160 nm and step edges below 80 nm are fabricated. These results emphasize the prospects of organic monolayers as high-resolution resists in rapid nonlinear femtosecond laser processing.     

Resonant infrared pulsed laser deposition of thin biodegradable polymer films

Thin films of the biodegradable polymer poly(DL-lactide-co-glycolide) (PLGA) were deposited using resonant infrared pulsed laser deposition (RIR-PLD). The output of a free-electron laser was focused onto a solid target of the polymer, and the films were deposited using 2.90 (resonant with O-H stretch) and 3.40 (C-H) μm light at macropulse fluences of 7.8 and 6.7 J/cm², respectively. Under these conditions, a 0.5-μm thick film can be grown in less than 5 min. Film structure was determined from infrared absorbance measurements and gel permeation chromatography (GPC). While the infrared absorbance spectrum of the films is nearly identical with that of the native polymer, the average molecular weight of the films is a little less than half that of the starting material. Potential strategies for defeating this mass change are discussed. Received: 22 August 2001 / Accepted: 23 August 2001 / Published online: 17 October 2001     

Characterization of particulates accompanying laser ablation of pressed polytetrafluorethylene (PTFE) targets

We present observations of sub-micron- to micron-sized particles generated by high fluence (≈2 J/cm²) 248-nm laser ablation of pressed polytetrafluorethylene (PTFE) targets in air at atmospheric pressure. The original target material was hydrostatically compressed ≈7 μm PTFE powder, sintered at 275 °C. Collected ejecta due to laser irradiation consists of four basic particle morphologies ranging from small particles 50–200 nm in diameter to larger particles ≈10 μm in diameter. Many particles formed in air carry electric charge. Using charged electrodes we are able to collect charged particles to determine relative numbers of ± charge. We observe roughly equal numbers of positively and negatively charged particles except for the largest particles which were predominantly negative. For a range of particle sizes we are able to measure the sign and magnitude of this charge with a Millikan-oil-drop technique and determine surface charge densities. The implications of these observations with respect to pulsed laser deposition of PTFE thin films and coatings are discussed. Received: 15 January 1999 / Accepted: 18 January 1999 / Published online: 7 April 1999     

Preparation of Y2O3:Eu thin films by excimer-laser-assisted metal organic deposition

Europium-doped yttrium oxide (Y₂O₃:Eu) thin films were successfully deposited on quartz and ITO/glass substrates by excimer-laser-assisted metal organic deposition (ELAMOD) at low temperatures. The effects of laser wavelength and thermal temperature on the films’ crystallinity and photoluminescence properties were investigated. Films irradiated by an ArF laser at 80 mJ/cm² and 400–500°C were highly crystallized compared with those prepared by thermal MOD. In contrast, when the film was irradiated by a KrF laser at 500°C, no crystalline Y₂O₃:Eu was formed. The Y₂O₃:Eu film irradiated by the ArF laser at 80 mJ/cm² and 500°C showed typical PL spectra of Eu³⁺ ions with cubic symmetry and a ⁵D₀→⁷F₂ transition at ∼612 nm. The PL intensity at 612 nm was much higher for the film prepared with ELAMOD than for that prepared by the thermal-assisted process, and the photoemission intensity of the film prepared with ELAMOD strongly depended on the substrate material.